The sun’s ultraviolet rays do in fact eliminate the free chlorine in your pool water on hot sunny days.
The addition of stabilizer will help prevent the sun’s ultraviolet rays from destroying the free chlorine in your pool water.
It is important to keep the stabilizer level between 30 and 50 ppm. 
This product must be used only when granulated chlorine is used (calcium hypochlorite) or in pools with a salt chlorinator.